Checking in the new AST node types

#12693 got merged 🎉.
It took a few rewrites essentially but I fell that the design of the
new nodes will allow us to scale with reasonable maintance cost when
adding new language specific nodes. The base class for new AST nodes
(Token) to sublcass from allows one to implement nodes in an
expressive manner by setting __slots__. The constructor of
Token then sets the .args of Basic based on __slots__
this has the benefit that you need not write setters and getters using
@property decorators (which quickly becomes tiresome when you have
many classes).

Checking in the refactored PythonPrinter and changes to lambdify

Finally the challenging work of refactoring lambdify got merged
into SymPy's master branch: #13046. We eventually decided
to drop the contents of the old translations dictionaries but leave
them be (in an empty state) in case users were modifying those in
their code. Hopefully this approach doesn't break any code out
there. Given how popular lambdify is among SymPy's users, it is a
bit worrying that the test suite is not that extensive. I do remember
a google engineer mentioning that the follow the "Beyonce principle":
"I you liked it you should have put a test on it". Funny at is may be
I hope I don't need to defend these changes with that arguement.